The present invention relates in general to a new and improved voltage regulator and more particularly to the power stage of a switching type power supply wherein the output voltage is obtained by opening and closing a power switching device, generally consisting of a semiconductor element. Regulated DC power supplies are known wherein a semiconductor element opens and closes the feeding circuit during consecutive time intervals. The ratio between the open and closed intervals determines the quantity of energy which is transferred during a unit time interval from a non-stabilized DC energy source to the output terminal in the form of a direct current at constant voltage.
The typical power stage of a stabilized voltage regulator, known to those skilled in the art, comprises at least a power transistor used as a switching device. The transistor, e.g. of the NPN type, has its collector connected to the positive terminal of a non-regulated voltage source, while its emitter is connected to the cathode of a commutating diode, also known as a "free wheeling" or "catch" diode. The anode of the aforesaid diode is connected to the negative terminal of the voltage source. A filter is connected in parallel with this diode comprising an inductance and a capacitor and the lead is coupled to the filter output. The base of the power switching transistor is triggered by an appropriate driving circuit.
In a circuit of the type described, the switching transistor is driven alternately to the conducting state (ON state) and to the non-conducting state (OFF state) by the output signal of the pulse type driving circuit applied to its base. Depending on the state of the switching transistor, it either passes or fails to pass current to the inductance. The function of the commutating, or catch, diode is to provide current to the inductance during the time interval when the switching transistor is in the OFF state. Thus, the current "commutates" between the transistor and the catch diode and its value is given by the average value between the minimum current flowing through the filter inductance corresponding to the rising edge of the driving pulse. The maximum current flowing in the inductance corresponds to the trailing edge of the driving pulse.
In a circuit configuration of the type described, high amplitude current spikes occur corresponding to each transition of the switching transistor from the non-conducting state to the conducting state. These spikes are due to the fact that the diode is a non-ideal element with respect to switching speed or reverse recovery.
In fact the diode, like all PN junction diodes, exhibits a phenomenon known as "minority carrier sweepout," such that after forward conduction, the reverse biased diode essentially constitutes a short circuit for a period of time TR, known as the recovery time. It follows, that during this period of time current spikes of high amplitude occur at the switching transistor and these are superimposed on the D.C. current supplied by the transistor. The peak currents, which occur during the recovery time TR of the catch diode, can reach very high values, e.g. up to five times the average forward current value.
In a voltage regulator of the type described, the existence of excessive spikes is subject to a number of disadvantages:
(a) The switching transistor must pass excessive currents, although only for finite periods of time. As a result, the transistor may be overstressed so as to cause a malfunction to occur, or to shorten its useful life. PA1 (b) The excessive short circuit current may upset the primary power supply of the circuit and cause noise in the rest of the system. PA1 (c) The efficiency of the switching regulator decreases with an increase in the frequency, as the interval during which excessive current flows becomes a significant portion of the transistor ON time.
The above-mentioned disadvantages are typical of known voltage regulators wherein the power switching transistor is driven by a suitable pulse signal and wherein the diode acts as a catch diode to provide current to the inductance when the transistor is in the non-conducting state.